This invention relates to a push-button switch and a keyboard, particularly to a stroke converting mechanism suitable for a push-button switch employing a so called a membrane switch.
Push-button switches are incorporated as inputting means in various electronic equipment. A typical usage of the push-button switch is in the keyboard of an electronic typewriter or that of an I/O (input/output) equipment in computer systems. Since the operator of such a keyboard is usually typing the push-buttons repeatedly for a long time, it is necessary to consider the design of a keyboard or its push-button switches from the stand point of not only efficiency but also human engineering.
The requirements for the push-button switches are: (1) adequate actuating pressure on the key top, desirably about 60 grams; (2) adequate stroke length of the key top, about 4 mm; (3) initial pressure threshold to prevent a failure input due to mistouching on the key top, about 20 grams; and (4) smooth sliding of the key top. In order to keep the smooth sliding of the key top, usually the surface of a slider, on which the key top is loaded, should provide a housing with a contact length of more than about 4 mm.
On the other hand, recent fashion in electronic equipment requires thin type keyboards, the so called low profile keyboards. In response to such requirements, a push-button switch or a keyboard incorporating a new switch called the membrane switch has been proposed. In the membrane switch, a set of make-break contacts is formed on the inner surfaces of two flexible insulating sheets which are separated by a spacer to face each other with a gap of a few tenths of a millimeter. The make-break contacts take the make position when one of the flexible insulating sheets is caused to sag by an external force given through the key top.
The membrane switch is advantageous for providing low profile keyboards and also for cutting the cost of keyboards, however, its small gap between the make-break contacts results in an undesirable key touch, if the stroke of the key top is directly transmitted to it. Therefore, a stroke converting mechanism is needed to make the membrane switches match the keyboards. The stroke converting mechanism converts a given stroke length of a key top of a push-button switch to a desired small displacement necessary for actuating make-break contacts like those in a membrane switch.
FIG. 1 is a cross-sectional view illustrating a push-button switch having a prior art stroke converting mechanism. Referring to FIG. 1, key top 1 is secured to slider 2 which is movably installed in housing 3, which has been secured to top panel 7. In the slider 2, a push rod 6 is movably inserted. When the key top 1 is free, the slider 2 and push rod 6 are lifted at their topmost positions by spiral springs 4 and 5. Below the bottom end of push rod 6, a set of make-break contacts 91 and 92 are placed to face each other with a distance of about 0.1 mm. The make-break contacts 91 and 92 are formed, for example, on the inner surface of a flexible insulating sheet 81 like polyester membrane and another insulating sheet 82, both of which are separated by spacer 8 and secured on the surface of base panel 10. The insulating sheet 82 is not required to be flexible, in general, and may be a rigid member like a printed circuit board.
In the situation as shown by FIG. 1, the key top is pushed up by the spiral spring 5, and pressed to the top of the housing 3. Accordingly, the bottom end of the push rod 6 is separated from the flexible insulating sheet 81 with a distance of about 1 mm, thus the make-break contacts 91 and 92 are in the break position. When the key top 1 is depressed with a sufficient force, the spiral spring 5 is compressed first, then the spiral spring 4 begins to shrink so as to balance the resetting forces of both springs. Until the push rod 6 touches the flexible insulating sheet 81, the ratio of the displacement of the slider 2 to that of the push rod 6 is determined by the spring constants of the spiral springs 4 and 5. When the foot of the push rod 6 touches the flexible insulating sheet 81 and initiates its deformation, the tension of the sheet 81 is incorporated in the resetting force against the key top 1, and after the make-break contacts 91 and, 92 take the make position, the restitution of the key top 1 depends on only the spiral spring 4. Thus, the stroke length (D) of the key top 1 is converted to the displacement (d) of the push rod 6.
The push-button switch having the stroke converting mechanism as shown in FIG. 1 requires a number of complicated parts, and therefore results in a high cost. Moreover, the stroke converting mechanism has difficulty in providing a low profile push-button switch or keyboard, because of the triple cylindrical structure comprising the housing 3, slider 2 and push rod foot 6, which inevitably leads to a voluminous structure of the housing 3. This suggests that, if the housing 3 has a structure so slim that its upper portion, at least, is contained in the key top 1, a low profile can be achieved while keeping the above mentioned contact length for eliminating the loose sliding of the key top. Furthermore, in the stroke converting mechanism as shown in FIG. 1, the external pressure applied on the key top 1 is directly transmitted to the make-break contacts 91 and 92. In other words, it is required to depress the key top 1 with a force at least equal to that necessary for actuating the contacts. This means that operators must depress the key top 1 with a force more than 100 grams, occasionally up to 200 grams. Such a large force gives the operators an unpleasant key touch and is apt to result in the physical symptoms well known as an occupational disease of keypunchers.